Color density analyzer



Aug 26, 1952 J. M. HALL 2,608,125

COLOR DENSITY ANALYZER Filed April 8, 1949 5 Sheets-Sheet 2 Aug. 26, 1952 J. M. HALL coLoR DENSITY ANALYZER Filed April e, 1949 5 sheets-sheet s 'I il L THRU FILTERS A REY MEDIUM REEN GREEN nes FLESH cHlLu INVENTOR.

Aug- 25 1952 J. M. HALL 2,608,125

l coLoR bENsITY ANALYZER Filed April 8, 1949 5 Sheets-Sheet 4 Aug. 26,- 1952 J, M HAL; 2,608,125

v COLOR DENSITY ANALYZER Filed April s, 1949 5 sheds-sheet 5 Patented Aug. 26, 1952 femore; xmii'afsriY *NLYZE Joseph Marian, (image1-IIL Application Ap'iir; 1949,'Sei'afoii'86340 liCIaims.

The principaly cib-jectfoithepresent inveritiori is to provide a methodand appara-tusf'fdrein'easur-f ing the amountfA light Y'refletted from 'a ffiven surface" or transmittedffthrouh a transparency, arid-'isparticulariyfsuitable Tordeterhiiiiig the color Hdoinponents andder'isiti'esfof -c'loredifs'urfaces and transparencies, including surfaeesto el'd`of"coior'photography. Y

It will be apparent atotice'ithattite-:pessime uses-"of :such -a lmethod 'and' apparatus f are eX-Y' tremelywide andvarie'd. -FioreXam-piefit maybe used* analyzing-'the color-values o`fpaints',-i'nks', dyes",rv varid vthe vlike, so r`that 1a Apieivi-cmsiyffc-i'oiued surface-or a'particiarpaiiityinkor dyemay be preciselyfdupli'cated. A l

A'The invention 'is *particularly "useful infthe field of color ,photography for accurate1y and rapidly `producing' `"Wel-1 balanced 'color Jprints. Due" to 'the' variance in'lconditons' 'wider-*Which cior transparenoies ''are-"taken it not' "always possilie" to" faithfuii'yreprddueet'tiiedcoiois' di' f the photographed 'o'biect' Jarid/or' to obtain the-most satisfactoryreproductionotheobject.

'-ih Vmaking positive-'polarprints-'ricainl impropf ei'iy" balanced -coiprtranspareneies'-it is possible', at I'eastirr somemeasure, fto'- neutraiizeaiifexce'ss of one color in the transparency byiinterposing suitablepl'orilters beweenthe'liht 'source and the V pririt paper'. Atthe `present "time l"the/dei termination "of the`v -proper""ci'or""`fi1tertouse-fis vjectedtcrtheiphotometerscreen from oneof said sources an amount 'representing'theecolor va-lue oi a Jgiven eo'ior" "found in thel surface ypr ftran'si parency "being ariiyzed and deterririnin"g thel amount er fiight'whieh is*riecessar'yftev rebaiaiiee f the .photometer .-Amove'i reiiective photometer filters between the light source I5 and the light transmitting spot I3; diaphragm means adjustable by a knurled knob I4 for controlling the illumination of the light transmitting spot I3; a means to indicate the effect of the illumination of the light transmitting spot I3 in terms of density, such means comprising a pointer I'I operatively connected to knob I4 and a dial I8 under the pointer carrying a suitable scale; a second diffusion glass 24 adjacent mirror I2 a lamp housing I9 containing a light source 20; a conversion filter 56 for correcting the color temperature of the light from light source to substantially 3200 K.; a filter head 2I for interposing color filters between the light source 20 and the colored surface to be illuminated; and an eye piece 22 focusable on the light transmitting spot I3 in the mirror I2 for observing the color relationship between the spot I'3 and the mirror area immediately surrounding the spot. The lamp housing I9 is removably connected to the densitometer casing I I by means of an arm 25 telescopically receivable into tube 26, the lamp housing I9 being vertically positionable with respect to the densitometer casing II by means of the knurled head set-screw 21.

A light source including lamp I5 in the densitometer casing II is controlled by a switch (not shown) and energized by power supplied through a light cord 29. As shown, the lamp I5 is held in a downward position, and the lamp socket 36 is secured to the densitometer casing II by suitable means, such as by a bolt 3I. The inner diusfon glass 23 and the conversion filter 55 are maintained in position by a retaining ring 80. The filter slide I6 is provided with apertures 32 for holding separation filters as desired. As best shown in Fig. 5, the filter slide I6 consists preferably of a panel 36 having a plurality of circular apertures 32 for receiving and holding color filters. Each of these apertures 32 is of substantially the same diameter as the light passage, and may be positioned selectively in registration therewith. As shown in Fig. 1, the filter slide I6 is preferably positioned in a receiving slot between the conversion filter 55 and the calibrated and adjustable aperture 33 for light control.

While the intensity of the light that reaches the light transmitting spot I3 from the lamp I5 in the densitometer casing II may be varied by any suitable means such as a gray scale or the like, I preferably employ a variable orifice in the path of the light from the lamp I5. As best shown in Fig. 3, the diaphragm means forming the variable orifice comprises a rearward plate 3'I and a forward plate 38 in overlapping relation. The rearward plate 31 has a rack portion 39 and two reentrant edges 4I conforming to logarithmic curves and positioned to form the upper half of a variable orifice. The forward plate 38 has an upper rack portion 40 and two curved edges 42 positioned to form the lower half of the orifice. The two rack portions 39 and 40 of the plates 3'I and 38 are engaged by a pinion 43 on a stub shaft 44 that carries an adjustment knob I4 whereby rotation of the knob in a counter-clockwise direction (as viewed from the front) will increase the area of the orifice 33, and rotation of the knob in a clockwise direction will decrease this area.

The dial I8 which is shown in Fig. 4 is faced with suitable scales for the measurements that are to be made. For example, the well known logarithmic scale of photographic density values may be used. The scales are traversed by a pointer I'I mounted on the stub shaft 44, the scales and the pointer II being calibrated with the illumination varying effect of the variable orifice means.

An extension of the densitometer casing II forms a partial light shield 34 for the mirror I2 for eliminating extraneous rays of light. In addition to this means of eliminating undesirable light rays there is provided an apertured disc 35 through which passes the reflected light from the material to be analyzed positioned at 28, as well as light from the light transmitting spot I3. The mirror I2 forming the densitometer face may be silvered on either of its surfaces for light reflection, and the light transmitting spot may conveniently have a diameter of the order of one thirty-second of an inch. v The eye piece 22 previously mentioned is rigidly connected to the base plate 45 by means of an arm 46, the arm being secured to the plate by means ofra screw 41, as shown.

Positioned vertically with respect to the densitometer casing II is a lamp housing I9, removably connected to the top of the densitometer casing II by means of the connecting arm 25, and whose position is controlled by a knurled head set-screw 2I positioned on the arm-receiving tube 26. The lamp housing I9 is preferably formed in two parts and held together by a screw5'I. Ilfhe lamp 20 within the housing I9 is held downwardly, and the lamp socket 56 is secured to the housing by means of a -bolt 5I. The lamp is energized by power supplied through a cord 52. A conversion filter 56 for correcting the color temperature of the light is positioned between the lamp 20 and the filter head 2I, as shown in Fig. 1.

The filter head 2|, best shown in Figs. 2 and 5, is connected to the lamp housing I9 by means of a plate 53. The plate 53 is secured to a side of the lamp housing I9 by means of a screw 54. As shown in Fig. 2, the apertured portion of the filter head is rotatable about an axis formed by the knurled head screw 49 so as to interpose the color filters between the light source 20 and the material to be analyzed when illuminated by light source 20.

As stated heretofore, the basic concept of the present invention consists in being able to analyze a colored object and determine the densities of the color components thereof. In applying this concept to the device embodied in my invention three types of color filters are used. These are: an A filter containing equal densities of magenta and yellow; a B filter having equal densities of cyan and yellow; and a C5 filter containing equal densities of magenta and cyan. For the purpose of this application, the colors magenta, yellow and cyan will be deemed pri-A mary colors.

The operation of analyzing the color values of a surface such as a sheet of paper, for example, to determine the densities of its component colors is performed as follows:

(l) Both light sources are turned on. all filters with the exception of filters 55 and 56 to standardize colortemperature are moved out of the way, and knob I4 is rotated counter-clockwise until the pointer I'I is at the zero density end of the dial. At this point aperture 3311s open the maximum amount. A sheet of 3200 K. white paper, hereinafter referred to merely 4as a'. white paper or a white surface, is placed on the easel immediately below aperture 28, setstrikes a surface having only a-vinagenta andf yellow coloring of the same density of Ythe A iilters, the density reading on the scale would be zero, indicating no cyan in the object being tested. If. however, this emanating light (containing no cyan) strikes a surface containing cyan, the density reading on the scale will indicate the density of the cyan only, since the magenta and yellow components of the two beams cancel out and have zero density by definition.

If a White light containing the primary colors strikes a surface which is spoken of as ycolored, the light rays of certain wavelengths will be reiiected. and rays of certain wave lengths will be absorbed. The maintenance of a certain color temperature of the light insures that the'light will contain certain proportions ofV light of various wave lengths, and 3200" K. is the standard generally used in photographic work, be-

cause at this temperature the density of the ma-4 genta, cyan and yellow are substantially equal.

When a B iilter is used, the light emanating from the filter contains no magenta, and upon striking a surface containing magenta, the instrument will indicate the density of the magenta only, since the yellow and cyan components of the B iilters are balanced to zero density by the interposition of such filters in both beams. Likewise with C5 ilters the color yellow is sub'- tracted from the light emanating therefrom (up to the rated capacity of the iilters), and the amount of yellow in the surface coloration will be indicated by an increased density reading equal to the density of the yellow in the surface, since the magneta and cyan in the first beam are balanced out by the corresponding C5 iilter in the other beam. In each instance the amount of loss in reflected light is balanced by closing aperture 33 to reduce the amount of light reaching spot I3 from light source I5. The amount of reduction that must be made in the lighting of the light transmitting spot I3 is indicative of the loss in reflection from the surface below aperture 28 and, thus, indicative of the amount of color present in the surface below aperture 28.

From the foregoing description certain modiiications will be apparent to those skilled in the art. For example, the calibrated diaphragm opening 33 might be placed in front of light source 20 and the initial balance (previously enumerated step (1)) obtained with the diaphragm opening substantially closed. Identical filters would then be placed in front of both light sources, and the colored surface to vbe analyzed placed in the plane immediately below aperture 28 (previously enumerated step (2)). reduction of the lighting on the portion of the face of the photometer immediately surrounding the spot would be made by increasing the opening of the diaphragm (in front of light source 20) until sufficient light had been added to make up for the loss resulting from the Acolor in the surface -being analyzed, which increase' would be a function of the amount of the particular color in that surface. A suitably calibrated scale would be provided in conjunction with the diaphragm opening mechanism to give readings of the amount of light that has been added, and hence the amount of color present in the surface. The scale could be substantially the same as that shown in Fig. 4, except that the 100 per cent reflection, zero density, end of the scale would indicate that the diaphragm opening 33 was of The measurement in thev 8 1 a minimum size rather than a Vxfr'iaximi'm size as in the illustrated embodiment.

The method and apparatus used in analyzing transparencies is substantially the same in theory lbut varies somewhat in actual application. rThe same analyzing instrument, as described vwith regard to Figs. l through 5., may be used, but the upper light source 20 is removed by loosening thumb screw 2'! and detaching arm 25 from tube 26. The analyzer is used in conjunction with a conventional enlarger, generally `6I, which serves as a projector; with the base 45 of the analyzer resting upon the enlarging easel I0, as shown in Fig. 7. The enlarger includes a suitable light source such vas bulb 52; preferably -the type of bulb is corrected to maintains. color temperature of 3200 KQ A negative holder 63 is included in the enlarger in a conventional manner, and a iilter holder such as slide 64' is also provided.` If desired, other types oi iilter holders may be used, such as that illustrated at 2i in Fig.v5. The enlarger also includes a lens system mounted in holder 65 and a suitable adjustable diaphragm in the lens holder vto vary the amount of light emanating from the projector.

The procedure in analyzing a color 4transparency follows the same-general pattern as that used in analyzing colored surfaces. A sheet of white 3200 K.paper is placed on easel I0 under aperture 28.' VLight source 62 and light source I5 are both turned on the filters, if any, are removed (except, of course, for the color temperature filters) knob I4 is rotated so that pointer I1 is at the left-hand end of dial I8, showing zero density. The nlm to be analyzed isA placed in the enlarger, focused, then removed. A balance is then obtained by adjusting the diaphragm opening of lens mounting 65 until the spot in the face `of the photometer disappears, at which time the instrument is in balance ready for use. The iilm to be analyzed is then again placed in the enlarger,

and the aperture 28 located over that portion ofthe projected picture, the color value of which is to be analyzed. Suitable pairs of filters are interposed in front of light sources I5 and 62 by moving filter holders I6 and 64 respectively. Knob I4 is rotated in a clockwise direction until a balance is again obtained between the illumination of spot I3 and the surrounding portions of mirror I2, whereupon a reading of color densities is taken from the outer scale of dial I8. As with the measurement of surface colors, if A filters are used, the density reading will be of the color cyan; if B filters are used, the density reading will be of the color magenta; and if C5 filters are used, the density reading will be of the color yellow.

As has previously been stated, the density read-` ings (the outer scale of dial I8) have been calibrated particularly for use in analyzing color transparencies. Assuming, for example, that the three readings obtained with a given transparency were: A lter equals 0.30, B filter equals 0.10, and

C5 iilter equals ,0.20, it would indicate the transparency had a density of 0.30 cyan, a density of 0.10 magenta, and a density of 0.2'0 yellow. From these gures one skilled inthe art will be able to determine the lters necessary to make a pleasingly balancedv positive color print.`

The theory of operation of the densitometer, when used in measuring the color values of transparencies, is based upon the' same subtraction process outlined in connection with measuring color values of colored surfaces. A balance is "whehrafsnecc colorghasbeen subtracted. thereihasbeencreatedftwolight lpaths to the face `of thefphotometer ,providing equal illumination thereupfll, ione pathff-rom thesenlarger light :62 tothe surface-to theface fand the 'other 4from the photometer jilight vI- to the back lighted spot. Atransparencyis interposed nin-the'irst of-these pathspand totheextent :that the-said :subtracted color :is :present ,in Ithe transparency, the light transmitted :therethrough will `be diminished.

For example, if a pair of A ltersis being used which @will not transmit the color r cyan (atleast tothezrateiicapacityof the lter)fand-a transparencyiisv interposed :whichs all cyan. (or t o put Litranotherfwam will onlyaztransmitf the color cyan) then substantially none of thelight from fthat ljghtsource will bepreceved ;a t mirror i2., l and thesdensityeading will be increased v'bythe yden- :sity ft-the cyan. Toit-he ;extentthata smaller Aamount of cyan A'is present iin the transparency, more light will 'i'be :passed therethrough 'and received atmirror t2 ,thereby givingsmallerdensity readings. As far as the theory or n.operation is concerned;itzmakesmo :diier'ence whetherft-he transparency is V.positioned aad-jacent the light source fwiththelter -'below the` transparency, as illustratedfin' 'Figf17,.1or whether thexilter is Vpositione'd on 'topwwith .the `'transparency i below it.

Tlf-fit should :be/desired to measure the color values of fa surface which 'has vbeencolored by meansfofprojected light, :suchas the projected image i of va transparency on an easel, while'at thersame timebeingrable to .viewthe colored image on the easel withoutany ofthe colorshaving been Ksubtracted "-therefrom; 'it may be done by means fof-a' varia-tion ofthecprocedure previously outlined for transparency. The initial balance maybe :obtained vin the :same Amanner yby using Va color temperature corrected white 3200`Klight .and fwith'fthe lters in Vholders IIS and vt[it and the transparency -in holder 'removed ifrom thel light jpaths. After lthe-balanceis obtained, thetrans- --parency is'moved'in-'front of lightsourcef52'by means vof negative Aholder iBS; and suitable `pairs `fo'fiilters'are inserted-in the two light pathsleadving to-the `face of thephotometen-the variation in procedure being 'thatinstead ofinsertingthe 'lterin the 'portion ofthe rst light path (that fpathfrorn projector light source to the plane at the'fbase'of aperture ZB-'an'd 'thence to the face of the photometer) between the projector light source and the-plane-at thebase of vaperture 28, "such-'asisillustratedinllig '7,y the `lter is inserted inthe'portionof the'same path between the plane Pat theibase v'of aperture V'28 andthe face of the g'photometer.

Fig. iillustrates an embodiment ofthe invenftion'bywhich thismay be performed. `A mount- `-ing-'I is-provided ontheffront of shield case 34,

which mounting is adaptedto receive alter slide fare -mounted 'The :mounting and slide permit *the/"filters to 'be :positioned in theiportion ofthe *tlightpathbetweenvthe lsurface at the base of fraperture 'L28 and theiz'face:of: the,photometer.

.'By' thismethodtheportion of thegimagegseen' -'68havingsuitablefaperture'69 inrwhich the filters 60 For iexample, .if the cyan `density in fa transpareny .is to :be measured, an A lter-will be insertedin'the second light :path between light source l5 and the photometer spot by means of holder lIt, andan lA lter will also be inserted, by means of holder t8, -in the portion ofthe first lightgpath between Vthe plane at the base of aperture 28 and the reflective face of the photometer. To Ythe extent that cyan light from the transparency isprojected to said plane andreiiected therefrom, it will besubtracted by the A;lter in :holder .68 before it reaches the face -of the photometer with acorresponding decreaseinthe amount oflightion the reilective portion of the photometer face. As in the vprevious methods, knob I4 willfbe rotatedto decreasethe amount of 1ight from ,source I5 to Ythe photometer spot, theyamount of such decrease :being readonthe suitable scale of dial I S, thusindicating thedensity ofthe subtracted color.

When making color Aprints 1 from color "transparencies, or when making black and-white prints, separation negatives, etc., the density-analyzer may be used to compute the correctfexposure times 'as Well -as used yto determine dens-ityvvalues for thediierent portions of the negativeand .the required color correction-filters necessary, in the case Aof colorwork, for a-pleasingly `balanced print. 'For this purpose it is necessary thatthe Alight source-15in the photometer be standardized not only as to 'Kelvin temperature, butalso as to intensity, andsuitable calibration means be available. In its preferred form thecalibration means is a circulan slide rule ztypewcomputer, vsuch as is; illustrated in Fig. 9. 1

Thecomputer consists'ofan outerjcircular dial, generally H and aninner circular dial, generally ,fi-2. The two dials are-concentricallymountednn pivot lpin 13,- andatleast one ofthe dialsfis rotat- ,able-withgrespect to the pin whereby the twadials may be rotated withrespecttoeach other. -f

IIhe outer dial I'il hastwo-scales thereupom-,the j "the portion Aof the transparency thatais Vthe fmo'st dense). For .thisV reason a heavy arrow 'I9 placed on :the inner .idial 'l2 :opposite the -'-der1sit'7 vreading'2'.2Cl.

VThe following is faxdescription of the: use `of',` the `density analyzer and exposure indicatordn making color prints from color transparencies. The

'density analyzer `is set `up fas sintfFig. d?, and' an :analysis ismadezof" the'.color components of the transparency. Normally .this is `vdone ziwithlthe aperture :28 A.positioned over the :most prominent "portionro'f the .projected image1' from thertansparency, or the Lmost -prominent portion of ithat part of the projected, image from whichaprint is to be made. Assuming that this `portion ofthe image is supposedttolbe'white orfslightlygray, and .that by .factual color analysis r:with-*the density :cussed .;exarnple,A that this portion ofuthe transparency had a density of 0.30 cyan, 0.10 magenta and 0.20 yellow, it would indicate that in making the print it was desirable to use additional filters having a density of 0.20 magenta and 0.10 yellow to obtain a balanced color density (the total densityof the transparency and filters then being 0.30 cyan, 0.30 magenta and 0.30 yellow), which would be a light gray.

1f, instead of selecting a white or gray portion of the picture for color correction, some other prominent feature were chosen, as, for example, a persons face, green grass or the blue sky, it is possible by referring to the chart in Fig. 6 to ascertain the color balance needed to obtain true color values, and the necessary corrective filters may be applied to provide the desired balance. Hence, if the green for grass in a transparency analyzes as .50 cyan, .50 magenta and .70 yellow, it would be apparent by referring to Fig. 6 that by applying corrective filters to add .30 cyan and .10 yellow to the projection light, the cyan and yellow densities would lbe equal and the magenta .30 density lower to give the desired ratio of colored densities.

With these additional filters mounted between the enlarger and the easel in a suitable holder (not shown) the operator is then ready to determine the correct exposure times for the type of printing paper which he plans to use. This may be done either with the enlarger diaphragm in lens holder 65 in a wide open position, or the diaphragm may be closed to the approximate point at which the operator believes a correct exposure will be obtained.

The base 45 of the density meter is moved until aperture 28 is positioned over the darkest area of the projected image, and knob I4 is rotated until spot I3 disappears as viewed through eye piece 22. The dark area density reading is then obtained from dial I8, which, by way of example, is assumed to be 2.40. Base plate 45 is then moved until aperture 28 is moved over to the lightest portion of the projected image, and knob I4 is again rotated until lspot I3 disappears. From the density scale cn dial I8 the bright area reading is obtained, which, by way of example, is assumed to be 1.30. This gives a density difference of 1.10

vReferring now to Fig. 9, it is assumed, for the purposes of the example, that the paper upon which the color print is to be made has an A. S. A. rating of 0.4 The outer dial 1I and the inner dial V12 are now rotated with respect to each other until the arrow 19, adjacent density reading 2.20, is aligned with the figure 0.4 on the A. S. A. exposure rating scale 15. The correct exposure time is now found on scale 14 opposite the dark area. reading just obtained. It must be remem- 'bered that the dark area density reading is always used unless a density difference correction is made, as hereinafter explained, in which case the bright area density reading may be used.

' `The dark area density reading having been 2.40, it will be notedthat the correct exposure time is 1.6 seconds, which is the number on scale 14 opposite the'gure 2.40 on scale 11. If the dark area. density reading had been 2.00, the exposure time would'be 5A; of a second; and if the dark area density had been 1.60, the exposurey time would be 1A; of a second, etc.

Y In some instances itmay be desirable to use the bright area density reading as a basis for computing exposure time. This might be desirable when a print is to be made from a very dense CFI negative or when the correction filters inserted have been of quite large rated values so that the light striking the easel is relatively small, or when it may have been necessary to close the diaphragm in lense holder 65 to a considerable extent in order that an accurate focus -is obtained over the Whole of the area to be printed, thus reducing the light striking the easel. In such instances it may be much easier to obtain an accurate balance in the bright area than in the dark area. l

Assuming that such conditions exit, the preferred procedure is to open the enlarger diaphragm the maximum amount and obtain the dark area and bright area density readings, as heretofore explained, which are assumed (for example) to be 2.40 and 1.30 respectively, with a density difference of 1.10. The enlarger diaphragm is now closed down until a sufficiently accurate focus is obtained over the whole of the projected image to be printed, and a new bright area density reading isobtained which is assumed to be 1.80.

Since the bright area density reading is to be used to obtain the exposure time, a density difference correction by means of scale 18 must be made. Assuming that the paper to be used for printing has a standard A. S. A. exposure rating of 5, dial 1I and dial 12 are rotated with respect to each other until the density difference of 1.10 on scale 18 is opposite the A. S. A. exposure rat ing of 5 on scale 15, as indicated in Fig. 9. The correct exposure is now found on scale 14 opposite the bright area density reading on scale 11. It will be seen that for a bright area density reading of 1.80, the proper exposure time is 1%, of a second.

The same readings should have been obtained if the dark area density reading had been used after the diaphrgam had been closed down, but it may have been considerably more difficult to obtain a balance when the dark area density reading was sought. With a density difference of 1.10 and a bright area density reading (after the diaphragm was closed) of 1.80, the dark area density reading at that time (had it been obtained) should have been 2.90. If the arrow 19 were set opposite the A. S. A. exposure rating of 5 on scale 15, it would be found that the density reading of 2.90 on scale 11 would be opposite 2/5 of a second on scale 14.

From the foregoing description it will be-seen that the density analyzer is extremely versatile. It not only may be used to obtain the density values of a colored surface, whether colored by projected light or pigments, dyes, or the like and as a basis for obtaining the correct colored filters for producing balanced color prints, but it also may be used to obtain the proper exposure time for those color prints.

This application is a continuation-in-part of my copending application, Serial No. 23,021, filed April 24, 1948, now abandoned, and application Serial No. 566,431, filed December 4, 1944, now issued as Patent No. 2,440,266, the disclosure of each of which is specifically incorporated herein by reference.

I claim: v I

l. In a device of the character describedfa first light source adapted to provide a light beam having a given color temperature, means for interposing color filters between said light source and an object whose color densities are to be determined, a mirror positioned to one side of the light beam and constructed and arranged to retween said light source and said object, means for converting the light from said light source to light of a given color temperature, a mirror for reflecting light from said object, said mirror having a light transmitting spot on its reilecting area, a second light source for illuminating said light transmitting spot from behind, means for interposing color filters between said light source and said light transmitting spot, means for converting the light from said second light source to light having the same color temperature as the converted light from said first light source, and variable means for illuminating said spot to known light intensities.

8. I'he device of claim 7 where the given color temperature of the light is substantially 3200 K.

9. The method of determining the density of a component primary color present in a colored. object which consists in directing a first beam of light having a given color temperature onto the object, placing a color filter containing equal densities of the other two primary colors in the path of said light prior to reaching said object, positioning a photometric screen adjacent to and above the object and in the path of light reflected from the object, projecting toward said screen for photometric comparison a second :beam of light having substantially the same color temperature as the first beam, placing a color lter containing equal densities of said other two primary colors in the path of the projected light prior to reaching said'screen, and then varying the intensity of said corrected and filtered projected light until the intesity of said projected light matches that of said reflected light to thereby obtain the component density of said rst mentioned primary color in said colored object.

10. The method as set forth in claim 9 in which the color temperature is substantially 3200 K.

11. The method of determining photometrically the density of one of a plurality of primary colors present in a colored object by the use of balanced light beams that have been standardized to the same known spectral distribution of energy, which consists in directing one of said .beams onto the object, positioning a photometric screen adjacent to and above the object and in the path of light from said one beam reflected from said object, projecting the second of said beams toward said screen for photometric comparison with said reflected light, removing from the light reaching the screen from the iirst beam the primary color the density of which in the object is sought to be determined, removing from the light reaching the screen from said second beam the same primary color, then varying the amount of light permitted to fall on the screen from one of said beams to bring the illuminanee from both beams into photometric balance, and measuring the'change in illuminance required to effect such balance.

12. The of determining photometrically the density of one of a plurality of primary colors present in a colored transparency yby the use of balanced light beams that have been standardized to the same known spectral distribution of energy, which consists in directing one of said beams through said transparency onto a surface, positioning a photometric screen adjacent to and above the surface and in the path of light from said one beam reflected from said surface, projecting the second of said beams toward said screen for photometric comparison with said reflected light, removing from the light reaching the screen from the first beam the primary color the density of which in the transparency is sought to be determined, removing from the light reaching the screen from said second beam the same primary color, then varying the amount of light permitted to fall on the screen from one of said beams to bring illuminance from both beams into photometric balance and measuring the change in illuminance required to effect such balance.

13. Apparatus for determining photometrically the density of one of a plurality of primary colors present in a colored surface by the use of balanced light beams that have been standardized to the same known spectral distribution of energy, said apparatus comprising a densitometer casing positioned adjacent to and above said surface and having an opening in its front wall, a photometric screen in said opening including a light-transmitting spot and a forwardly-facing mirror adjacent to the spot, a light projection housing mounted above the densitometer casing and having an opening in its lower Wall, light means for projecting a first beam of light downwardly from said housing to said surface from whence it is reflected to the front face of said photometric screen, and for projecting a second beam of light to the rear face of said photometric screen, means for adjusting said two beams of light to the same color temperature, means for subtracting given spectral bands of light from the light in each of said beams, and calibrated means for varying the amount of light permitted to fall on said screen from one of said beams, whereby the beams may be brought into ,photometric balance.

14. Apparatus as set forth in claim 13 in which the light projection housing is mounted on'the densitometer casing and is vertically adjustable relative thereto.

15. Apparatus as set forth in claim 13 in which the light projection housing comprises an enlarger having a transparency holder and a lens for projecting the image of the transparency to said surface.

16. Apparatus as set forth in claim 13 in which the calibrated means for varying the amount of light permitted to fall on the photometric screen from one beam comprises a variable light-transmitting orifice.

JOSEPH M. HALL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,958,679 Reiss May 15, 1934 2,234,278 Richter Mar. 11, 1941 2,253,356 Van den Akker Aug. 19, 1941 2,255,034 Bauer Sept. 2, 1941 2,294,876 Walker Sept. 1, '1942 2,388,842 Hanson Nov. 13. 1945 2,442,506 Morris June 1, 1948 2,461,464 Aronstein Feb. 8, 1949 

